In known common rail fuel systems, a high pressure fuel pump is arranged to charge an accumulator volume in the form of a common rail with fuel at high pressure for delivery to a plurality of associated injectors. Each injector includes a valve needle which is movable by means of an actuator, towards and away from a valve seat, to control fuel injection through a plurality of injector outlets.
It is known to control valve needle movement by means of an electromagnetic actuator including a solenoid winding through which a current is passed to activate an armature. In turn, the armature controls a servo valve for controlling a control pressure applied to the valve needle and, hence, valve needle movement. It is also known, however, that particularly good injector performance can be achieved by using a piezoelectric actuator to drive movement of the valve needle. The piezoelectric actuator includes a stack of piezoelectric elements to which a voltage is applied to extend and contract the stack length. The actuator may be coupled directly to the valve needle so that, as the stack is retracted, the injector valve needle is caused to move with the stack retraction. Alternatively, the stack may be coupled to the valve needle via a motion amplifier (for example, a hydraulic amplifier). In other injectors the piezoelectric actuator controls valve needle movement indirectly through a servo valve.
One example of a piezoelectrically operable fuel injector is described in our granted European patent EP 0995901. Here, the piezoelectric actuator is coupled directly to the valve needle through a coupler having both hydraulic and mechanical coupling elements to provide variable amplification of movement of the valve needle.
Piezoelectric actuators provide a particular benefit over solenoid injectors as they are capable of generating high rates of force change which gives fast needle response. Injectors configured with direct acting piezoelectric actuators are particularly beneficial in this regard. However, one problem with using a direct acting piezoelectric actuator is that a greater mechanical force is required from the actuator in order to move the valve needle. Such high forces, and the associated high rates for force switching, are transmitted through the injector to the associated engine and result in an undesirable level of noise generation from the engine structure.
It is an object of the present invention to provide an engine in which the injectors can offer the benefits of a fast acting, high force actuator but in which the level of noise generation within the engine is substantially reduced.